It's almost always used, and those threads never amount to anything because they can't. That's the point. The rule doesn't work. Things not working as intended by the creators is already happening when you ask the question, since they weren't intending for their 'verse to interact with another beyond their own stories or they would have given information on how to deal with that, so that concern is immaterial.

Any middle ground would have to be a third universe capable of accomodating beings from both. But that would necessarily mean that the entire conflict happens based on the rules of that third universe, not the two that are conflicting.

Having both sides use their own rules just means they can't interact, at least not in an argument, since there is no way to resolve what happens when they clash. (Would magic even work on RL humans? What happens when a bullet from a world were magic does not exist hits a magic shield? Can it even hit the shield?)
You haven't eliminated the What-ifs. You removed any ability to resolve them.

While the authors never intended for one verse to interact with another, they do establish laws that their worlds follow. The world-building, the threats, the characters and such revolve around those laws. As I said, if you forcibly change it to another, those characters wouldn't be what they are anymore. Some might become much more powerful, some much less. A WoW fire elemental, for example, is intended to be susceptible to being damaged by pure physical attack without any magic involved. If you force it to go with our real life physical laws, suddenly it turns into a different kind of being that is night invulnerable against most conventional weapons, which means it'd be able to massacre most of Azerothian armies except heroes or mages (who are the minority). Or Sargeras, if we are to go with our natural laws, he would become a much bigger threat than the massive one he already is in WoW - a flaming being that big would have scorched the planet, screwing up the tides and various other cataclysmic effects just by standing next to it, which he clearly can't do with WoW physical laws as seen in the cinematic. I'm not the OP, but I don't think that's what he wanted to compare with Earth armies. It's more likely that he was aiming for a situation of our armies as we know it against WoW threats as we know them,

In regards to the interactions, going with the assumption that I mentioned: why wouldn't magic work on RL humans? We don't have any innate magic resistance (our physical laws), so magic should work on us just as fine (WoW physical laws) as it does on any normal humans (or Orcs, Elves, Gnome, etc.) in WoW. Why shouldn't a bullet be able to hit a shield if a normal arrow (not an enchanted or magical one, just normal arrows - the type normal soldiers - i.e: the NE sentinels - uses) can? It's one thing if WoW's blades / arrows / guns are all infused with magic, or all characters used magics in their attacks - but that wasn't the case. In both WoW and real life, those WoW arrows or our bullets are essentially just projectiles. If a normal projectile work against a magic shield (whatever kind of thing a magic shield is categorized as) in one setting, why shouldn't something that is basically the same, but more powerful, wouldn't? There, I explained two possible interactions while keeping things within their established world-building settings.

I do see your points, but I just wanted to mention that it's how these kinds of threads normally work. There are people who like reading them, there are people who don't. There are people who enjoy them as a way to articulate the mechanics of stories, and there are people who find them absurd. In the end, it's a subjective matter so I wouldn't argue about it, but I want to point out that these kinds of threads wouldn't end up with much results if we are to force one verse to go with another verse's physical laws. The stories' world-building wouldn't be what they are any longer in that case. A simple "things wouldn't be as we know it, there are too many things would change" doesn't make good nor interesting a "discussion", now does it? :P I guess it's the kind of "follow its rule or leave it" kind of threads (and this line sounds more aggressive than I intended it to be, putting this tiny note in just so there wouldn't be any misunderstanding of the tone).